1. Field of the Invention
The present invention relates to a television tuner. In particular, the present invention relates to the layout in a television tuner suitable for a dual-screen television receiver for splitting an input television signal into two television signal components, one of which is converted to an intermediate-frequency signal component and the other of which is output externally.
2. Description of the Related Art
FIG. 2 is a schematic plan view showing the layout of component blocks of a known television tuner. The known television tuner includes a box-shaped metal frame 51 having a first side plate 51a; a second side plate 51b opposite to the first side plate 51a; a third side plate 51c; and a fourth side plate 51d. The television tuner further includes a first shield plate 52; a second shield plate 53; a third shield plate 54; and a fourth shield plate 55. The shield plates 52 to 54 are disposed parallel to one another in that order from the third side plate 51c to the fourth side plate 51d, and bridge the side plates 51a and 51b. The fourth shield plate 55 bridges the shield plates 53 and 54.
The area inside the metal frame 51 is divided into a first compartment 56 between the third side plate 51c and the first shield plate 52; a second compartment 57 between the first shield plate 52 and the second shield plate 53; a third compartment 58 enclosed by the first side plate 51a, the second shield plate 53, the third shield plate 54, and the fourth shield plate 55; a fourth compartment 59 enclosed by the second side plate 51b, the second shield plate 53, the third shield plate 54, and the fourth shield plate 55; and a fifth compartment 60 between the third shield plate 54 and the fourth side plate 51d. 
In the metal frame 51, the shield plates 52 to 55 stand upright (i.e. the height direction is in the plane of the paper, from the second side plate 51b to the first side plate 51a) and circuit boards (not shown) for mounting various circuits are disposed in the compartments 56 to 60.
The third side plate 51c has a first connector 61, which is a television-signal-receiving connector, mounted thereon at a portion adjacent to the first side plate 51a. The first side plate 51a has a second connector 62, which is a television-signal-outputting connector, mounted thereon at a portion interfacing with the first compartment 56. The second side plate 51b has a plurality of terminals 63 mounted in line thereon such that the terminals 63 are separated from one another at substantially identical intervals.
The reason for the first connector 61 being disposed adjacent to the first side plate 51a is to ensure a sufficient height of the first connector 61 from the second side plate 51b when the motherboard (not shown) of a television receiver is connected to the terminals 63.
The first compartment 56 contains a first wideband amplifier 64 for amplifying an input television signal; a distributor 65 for splitting the television signal into two television signal components; and a second wideband amplifier 66 for amplifying one of the two television signal components. The first wideband amplifier 64 is disposed close to the first connector 61. The distributor 65 is disposed between the second connector 62 and the first wideband amplifier 64 such as to be close to the second connector 62. The second wideband amplifier 66 is disposed opposite to the distributor 65 with respect to the first wideband amplifier 64; in other words, the second wideband amplifier 66 is disposed adjacent to the second side plate 51b. 
The second to fifth compartments 57 to 60 contain circuits constituting a tuning section. In detail, the second compartment 57 contains an input tuning circuit 67, which includes a tuning circuit for the UHF and VHF bands. The input tuning circuit 67 has its input terminal disposed adjacent to the first side plate 51a. 
Underneath the second shield plate 53 (on the bottom surface of the circuit board) are provided a UHF amplifier 68 and a VHF amplifier 69. The third compartment 58 contains a UHF-band interstage tuning circuit 70 and the fourth compartment 59 contains a VHF-band interstage tuning circuit 71. The fifth compartment 60 contains an oscillator 72 and a mixer 73.
In the known television tuner described above, a television signal input to the first wideband amplifier 64 via the first connector 61 is split into two television signal components in the distributor 65. One of the two television signal components is input to the input tuning circuit 67 via the second wideband amplifier 66. The television signal component is converted to an intermediate-frequency signal in the mixer 73 and is finally output from the terminals 36. The other of the two television signal components split in the distributor 65 is output from the second connector 62 to another television tuner.
In the known television tuner described above, the distance between the third side plate 51c and the first shield plate 52 is small, i.e., the area of the first compartment 56 is small. For this reason, the second wideband amplifier 66 is disposed opposite to the distributor 65 with respect to the first wideband amplifier 64; in other words, the second wideband amplifier 66 is disposed adjacent to the second side plate 51b. Thus, when a television signal output from the first wideband amplifier 64 is passed to the distributor 65 and one of the two television signal components split in the distributor 65 is input to the input tuning circuit 67 via the second wideband amplifier 66, the television signal travels first in one direction, then in the opposite direction, and again in the one direction. This causes the distributor 65 to have its input and output terminals poorly isolated from each other and the second wideband amplifier 66 to have its input and output terminals poorly isolated from each other, resulting in the television signal entering the input tuning circuit 67 being poorly isolated from the television signal output to the second connector 62. Thus, channel switching undesirably causes noise to be mixed with another, externally connected tuner.